224 THE REALITIES OF MODERN SCIENCE 



We can conceive of three such volumes if we take 

 into account that the molecular mass may be either 

 a gas, a liquid, or part liquid and part gaseous. When 

 a gas changes into a liquid we say it condenses, and 

 conversely when a liquid changes into a gas or vapor 

 we say it boils. This reversible phenomenon we shall 

 need to discuss before considering further the roots of 

 Van der Waals's equation. 



The temperature at which this phenomenon occurs 

 is the boiling point. The boiling point is raised if the 

 liquid is subjected to an increased pressure. Water 

 boils at 100 C. under a pressure of 76 cm. of mercury. 

 In mountain districts, the pressure being less, boiling 

 occurs at correspondingly lower temperatures. In a 

 boiler of a steam-engine plant the boiling point of 

 water may be very much higher than 100 C. At the 

 boiling point, however, the pressure exerted by the 

 water vapor molecules is always just equal to that of 

 the air or vapor molecules above them. If the mole- 

 cules are not allowed to escape from the boiler, as is 

 the case in " getting up steam," the increase of mole- 

 cules in the space above the liquid results in an increase 

 in the pressure against which those still in the liquid 

 must escape. They must, therefore, have a higher 

 kinetic energy, and when they do escape they increase 

 still further the pressure exerted upon the liquid by 

 its own vapor. The pressure of the steam (and water) 

 thus rises. When it reaches the desired value steam 

 may be withdrawn to run the engine, but the molecules 

 thus withdrawn must be continuously replaced by others 

 with the same average k.e. or the pressure will fall. 



If the pressure exerted on a body of vapor in contact 



